Field
Exemplary embodiments of the invention relate to a vehicle lamp for use in an automobile or the like, and in particular relate to an overcurrent protection circuit.
Related Art
Hitherto, halogen lamps and high intensity discharge (HID) lamps have prevalently been used as light sources for vehicle lamps, and in particular for headlamps. Recently, the development of semiconductor light sources, such as light emitting diodes (LEDs) is progressing as an alternative.
For example, JP 2004-140885 A (corresponding to US 2004/0070374 A1) describes a vehicle lamp that includes a laser diode (also referred to as a “semiconductor laser”) and a phosphor instead of LEDs in order to further improve visibility. In JP 2004-140885 A, the phosphor is irradiated with ultraviolet light that is excitation light emitted from a laser diode. The phosphor generates white light upon receiving the ultraviolet light. The white light generated by the phosphor is emitted ahead of the vehicle lamp, to thereby form a specific light distribution pattern.
FIG. 1 is a circuit diagram of a vehicle lamp 1r studied by the inventor. A light source 2 includes a laser diode 3. A lighting circuit 10r includes a boost converter (step-up DC/DC converter) that receives and steps up a power source voltage VBAT from a battery. A drive circuit 20r includes an inductor L21, a switching transistor M21, a rectifier diode D21, and an output capacitor C21. A controller 22 executes feedback control for the duty ratio of the switching transistor M21 so that a current ILD flowing into the laser diode 3 matches a target current.
The laser diode 3 has little ability to withstand overcurrent. There is a concern that the reliability of the laser diode 3 may deteriorate if supplied with an overcurrent. Overcurrent may occur in the vehicle lamp in, for example, the following circumstances.
In consideration of maintainability, there is a case where the laser diode 3 is connected to the lighting circuit 10r so as to be replaceable. More specifically, the laser diode 3 may be connected to the lighting circuit 10r through a connector. There is a concern that if a connection point of the connector might fluctuate between a contact state and a non-contact state (chattering), charge stored in the output capacitor C21 of the drive circuit 20r might flow into the laser diode 3 at the moment when the connection point is restored to the contact state, which may generate an overcurrent.
In a normal state without disturbance, the drive circuit 20r performs a switching operation at a given fixed duty ratio. When the power source voltage VBAT increases rapidly, it is necessary to lower the duty ratio immediately in order to keep the drive current (lamp current) ILD flowing into the laser diode 3 constant. However, there is a concern that due to delay in the feedback loop, switching takes places at a large duty ratio immediately before fluctuation of the power source voltage, excessive energy is stored in the inductor, and energy is supplied to the laser diode 3 as an overcurrent.
Such issues do not only arise in boost converters, but may also arise in a circuit system which drives the laser diode 3 using a power source having a topology including an inductor or a transformer, and an output capacitor, such as a buck converter (step down DC/DC converter), a Cuk converter, a Zeta converter, a flyback converter, or a forward converter. Similar issues may also arise in a circuit system that drives the laser diode 3 using a linear regulator. Overcurrent protection is also important in situations where an LED is used as the light source 2 instead of a laser diode 3.